Driver device for a voltage-controlled oscillator

ABSTRACT

A driver device for a voltage-controlled oscillator, having an unstable voltage source, a voltage regulator, a driver for generating a control voltage for the oscillator, and a feedback loop which controls the driver as a function of the output signal of the oscillator; the voltage regulator supplying the feedback loop with operating voltage while the driver is powered by the unregulated voltage of the voltage source, and the feedback loop compensates for voltage fluctuations of the voltage source with the aid of the driver.

FIELD OF THE INVENTION

The present invention relates to a driver device for avoltage-controlled oscillator, having an unstable voltage source, avoltage regulator, a driver for generating a control voltage for theoscillator, and a feedback loop which controls the driver as a functionof the output signal of the oscillator. The present inventionparticularly relates to a driver device for a microwave oscillator, e.g.a Gunn diode, of a radar system which is designed to be installed inmotor vehicles.

BACKGROUND INFORMATION

In motor vehicles, radar systems are used for measuring the distancesand relative speeds of preceding vehicles, so that automatic speedcontrol and distance control (ACC; Adaptive Cruise Control) can beimplemented. The frequency of the microwave radiation generated by theoscillator is controlled, using the voltage applied to this oscillator.This frequency must often be modulated in the course of the radarmeasurements. For example, in the case of FMCW (Frequency ModulatedContinuous Wave) radar, frequency is modulated using different frequencyramps, so that the differential frequency (difference in frequency)between the emitted radar signal and the radar echo becomes a functionof the propagation time. The distances of objects located by the radarsensor can then be determined by evaluating the differential frequencyspectrum.

For a high measurement accuracy, it is necessary that the frequenciesgenerated by the oscillator correspond to the modulation signal withhigh precision. For this purpose, the frequency of the oscillator iscontrolled with the aid of the feedback loop and the driver in a closedcontrol loop. In known systems, the feedback loop is a frequency-lockedloop (FLL; Frequency Locked Loop). In this case, the modulation signalis a voltage signal, which indicates the setpoint frequency value validfor the specific time. The frequency of the oscillator is also convertedinto a voltage and compared to the modulation signal, and the driver iscontrolled on the basis of the comparison result.

In motor vehicles, the vehicle battery is available as a voltage sourcefor the driver device. However, the problem with this is that thisvoltage source is unstable, since, depending on the charge of thebattery, the battery voltage is subjected to more or less sharpfluctuations when switching other loads in the vehicle on or off. Inorder that the functioning of the driver device and the oscillator isnot affected by such voltage fluctuations, a voltage regulator, whichsupplies both the driver and the feedback loop with a regulated and,therefore, stable operating voltage, has been provided, till now,between the vehicle battery and the driver.

However, the known set-up has the disadvantage that a certain, minimumvoltage drop inevitably occurs at the voltage regulator, so that the DCvoltage available to the driver and the oscillator is considerably lowerthan the battery voltage. Therefore, when the voltage of the vehiclebattery is low, there may not be sufficient voltage available foroperating the oscillator, and consequently, the radar system must beswitched off.

SUMMARY OF THE INVENTION

An object of the exemplary embodiment and/or exemplary method of thepresent invention is to provide a driver device, which allows a highertolerance to a low battery voltage.

The object of the exemplary embodiment and/or exemplary method of thepresent invention is achieved in that the voltage regulator supplies thefeedback loop with operating voltage, while the driver is powered by theunregulated voltage source, and that the feedback loop is designed tocompensate for voltage fluctuations of the voltage source with the aidof the driver.

In this design approach, the inevitable voltage drop at the voltageregulator therefore does not affect the voltage available to the driver,so that even when the battery voltage is low, a sufficient voltage isstill available to the driver and the oscillator. In practice, thethreshold value for low-voltage shutdown may thus be reduced byapproximately 0.5 V, so that a correspondingly higher availability ofthe system is achieved.

To operate the feedback loop and other electronic components, such asthe modulation circuit and a controller for the radar system, arelatively low voltage of, e.g. approximately 5 V is generally required,while the oscillator requires a higher voltage of, e.g. approximately 8V. The design approach according to the exemplary embodiment and/orexemplary method of the present invention has the advantage that thevoltage regulator only needs to provide the lower voltage of 5 V, andthat a structural simplification and cost savings are achieved byomitting a separate voltage regulator for the driver and the oscillator.

In the driver device of the present invention, the already presentcontrol function of the feedback loop and the driver alone ensures thatthe fluctuations of the battery voltage do not affect the oscillator. Tothis end, the circuit must only be designed in such a manner, that thevoltage drop at the driver causes the battery voltage, which may be 12 Vor more in the case of a fully charged battery, to be reduced to thevoltage of approximately 8 V required for operating the oscillator, andthat the feedback loop has a sufficiently low control delay as afunction of the required frequency accuracy.

The feedback loop may take the form of a phase-locked loop (PLL). Anintermediate-frequency signal is formed from the microwave signalgenerated by the oscillator, by mixing the microwave signal with areference signal having a fixed frequency; the frequency of theintermediate-frequency signal being equal to the difference between themicrowave frequency and the reference frequency. The PLL compares thephase of the intermediate-frequency signal to the phase of a referencesignal, whose frequency corresponds to the desired microwave frequencyand, if indicated, is modulated according to the utilized measuringprincipal (e.g. FMCW). The PLL then forms a control signal for thedriver on the basis of the phase difference. Since the phase differencereacts (responds) very critically to deviations in frequency between theintermediate-frequency signal and the reference signal, the phase-lockedfeedback loop allows any deviations of the microwave frequency from thesetpoint value represented by the reference signal to be rapidlycorrected (compensated for), in particular, even when such deviationsare caused by fluctuations in the battery voltage.

A filter circuit, which may also include a reverse-polarity protectioncircuit, but not a voltage regulator, may be provided between thevoltage source and the driver. Since such a filter circuit functions ina nearly non-dissipative manner, the voltage drop caused by it isnegligible.

In a modified specific embodiment, the filter circuit includes a voltagelimiter, which limits the battery voltage, which can be up to 17 V, to avalue of approximately 9 to 10 V. In this manner, the power loss isdivided between the driver and the filter circuit, when the batteryvoltage is very high. However, when the battery voltage falls below thelimiting value of 9 or 10 V, the current limiter becomes inactive(inoperative), and only the voltage drop at the driver remains.

With the exception of the battery, the components of the driver devicemay optionally be integrated to form one unit or designed separately tohave a modular construction.

A radar system for motor vehicles, which is provided with theabove-discussed driver device, is described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified circuit sketch of an oscillator and acorresponding driver device.

FIG. 2 shows a circuit sketch of an oscillator and a driver deviceaccording to a modified exemplary embodiment.

FIG. 3 shows a time-dependency diagram for explaining the method offunctioning of the driver devices.

DETAILED DESCRIPTION

FIG. 1 shows a voltage-controlled oscillator 10, a corresponding driverdevice 12, as well as a voltage source, which takes, in this case, theform of a battery 14 of a motor vehicle. In the example shown,oscillator 10 is a Gunn diode of a microwave radar, which is used in themotor vehicle to measure distance.

Driver device 12 includes a filter circuit 16, a driver 18 (transistor),a feedback loop 20 taking the form of a phase-locked loop (PLL), avoltage regulator 22, and, in the example shown, a controller 24 aswell, which controls the functions of feedback loop 18 and oscillator 10and, if indicated, those of other components of the distance-measuringsystem.

Filter circuit 16 is connected to battery 14 and is used, first andforemost, to filter out interference signals that could be coupled invia the line coming from the battery. In addition, the filter circuithas the function of protecting against polarity reversal. Batteryvoltage Ub is filtered by filter circuit 16, but is transmittedunregulated to driver 18. Therefore, practically no voltage drop occursin filter circuit 16.

Driver 18 is activated in such a manner by a control signal S generatedby feedback loop 20, that a specific voltage drop U1 occurs at thedriver, and the driver supplies oscillator 10 with a control voltage U2that is equal to the difference between Ub and U1. The level of controlvoltage U2 determines the frequency of microwave signal RF generated byoscillator 10. At the same time, control voltage U2 constitutes theoperating voltage of oscillator 10 and consequently provides the energy,which the oscillator requires for producing the microwave radiation.Control voltage U2 is typically on the order of approximately 8 V.

The frequency of microwave signal RF generated by oscillator 10 iscontrolled in a closed control loop and modulated as required. For thispurpose, microwave signal RF is extracted and mixed in a mixer 26 with areference signal B, which has a fixed frequency and is generated by afurther oscillator, e.g. a dielectric resonator 28. The output signal ofmixer 26 is an intermediate-frequency signal IF, whose frequency isequal to the difference between the frequency of the microwave signaland the frequency of reference signal B. This intermediate-frequencysignal IF is supplied to feedback loop 20.

Furthermore, feedback loop 20 receives a reference signal Ref having avariable frequency, which is of the same order of magnitude as thefrequency of intermediate-frequency signal IF. Feedback loop 20 comparesthe phase of intermediate-frequency signal IF to the phase of referencesignal Ref and generates control signal S for driver 18 on the basis ofthe comparison result. In this manner, the frequency of oscillator 10 iscontrolled so that a fixed phase relationship is maintained betweenintermediate-frequency signal IF and reference signal Ref. Therefore,the frequency of microwave signal RF is rigidly coupled to the frequencyof reference signal Ref, with very little control delay.

Controller 24 controls a modulator 30 which, on its part, varies thefrequency of reference signal Ref, so that the microwave signal ismodulated in the desired manner.

Voltage regulator 22 supplies a controlled operating voltage of 5 V tofeedback loop 20 and controller 24 (as well as to modulator 30, ifindicated) and is powered, on its part, by the output voltage of filtercircuit 16. In addition, a smoothing capacitor 32 is connected betweenthe output of filter circuit 16 and ground.

The exemplary embodiment shown in FIG. 2 differs from theabove-described exemplary embodiment in that the main components of thedriver circuit take the form of separate components, which may also besituated on different printed circuit boards. In addition, a filtercircuit 16′, which simultaneously has a voltage limiting function andlimits the voltage of battery 14 to a specific maximum value on theorder of 9 to 10 V, is provided in place of filter circuit 16.

Therefore, when battery voltage Ub is above this maximum value, anadditional voltage drop U1′ occurs at filter circuit 16. This has theadvantage that the power loss is divided up between driver 18 and filtercircuit 16′, so that higher voltages of battery 14 may also betolerated. However, filter circuit 16′ also does not have a controlfunction in this case, and voltage drop U1′ decreases to 0 when thebattery voltage falls below the limiting value of 9 to 10 V. Therefore,a low battery voltage may be tolerated, as long as the battery voltageis greater than the sum of control voltage U2 and residual voltage dropU1, which is also inevitable when the driver is controlled (activated)at a maximum level. The controller and the modulator are not shown inFIG. 2.

FIG. 3 illustrates the control function of the driver device accordingto FIG. 1, using as an example an operating phase, in which frequency frof microwave signal RF is modulated with a decreasing ramp, as is shownin the frequency/time graph in the lower part of FIG. 3. At time t1,battery voltage Ub decreases from a value greater than 10 V to a valueof less than 9 V, because, for example, an additional load was switchedon in the vehicle. Without frequency regulation, this voltage drop wouldlead to a corresponding decrease in control voltage U2 and,consequently, to a decrease in frequency fr. However, this voltage dropis instantaneously compensated for by feedback loop 20. Control signal Sincreases, so that voltage drop U1 at driver 18 decreases and controlvoltage U2 is essentially maintained at the previous value. At the sametime, control signal S ensures that the phase relationship betweenintermediate-frequency signal IF and reference signal Ref is retained,so that control voltage U2 and, therefore, frequency fr of the microwavesignal as well, decrease with the frequency of reference signal Ref, inaccordance with the ramp function.

The above explanation of the functioning method also applies analogouslyto the exemplary embodiment according to FIG. 2.

1. The driver device for a voltage-controlled oscillator, comprising: anunstable voltage source; a voltage regulator; a driver to generate acontrol voltage for the oscillator; and a feedback loop to control thedriver as a function of an output signal of the oscillator; and whereinthe voltage regulator supplies the feedback loop with operating voltage,while the driver is powered by the unregulated voltage of the voltagesource, and the feedback loop compensates for voltage fluctuations ofthe voltage source with the aid of the driver, and wherein the feedbackloop receives an intermediate-frequency signal, which is formed bymixing the output signal of the oscillator with a reference signalhaving a fixed frequency, and the feedback loop compares the phase ofthe intermediate-frequency signal to the phase of a reference signal andcontrols the driver based on the comparison result so that a frequencyof the oscillator follows a frequency of the reference signal.
 2. Thedriver device of claim 1, wherein the feedback loop includes aphase-locked loop.
 3. The driver device of claim 1, wherein the controlvoltage for the oscillator is greater than the operating voltage for thefeedback loop supplied by the voltage regulator.
 4. The driver device ofclaim 1, wherein a filter circuit is inserted between the voltage sourceand the driver.
 5. The driver device of claim 4, wherein the filtercircuit includes a voltage-limiting function.
 6. The driver device ofclaim 4, wherein at least the filter circuit and the driver take theform of separate components.